The invention relates to a solid state microwave source of the type incorporating a plurality of negative resistance diodes operating in parallel and in particular to such a source able to operate in millimeter wave bands.
The extreme frequency bands of the radiofrequency spectrum corresponding to millimeter and submillimeter wavelengths can be used for transmitting electrical signals characterised by a high information flow or a wide band. In addition, the waves in these extreme frequency bands at least over a short distance facilitate the construction of discrete links. Solid state microwave sources are known in which the active element is either an FET (field effect) transistor or a GUNN-type diode and more generally of the IMPATT type in the higher frequency bands. The output power supplied by these active elements decreases relatively rapidly when the oscillation frequency increases. For example, in the case of IMPATT diodes, the unitary output powers are approximately a few watts at 15 GHz, 1 watt at 50 GHz, 150 mW at 90 GH and 3.5 mW to 150 GHz.
These output power levels are below what is required by users. Thus, the solution of the problem of providing microwave sources with a higher power level consists of combining a plurality of active elements which have to oscillate synchronously
In the case of microwave transmission lines it is known that a rectangular waveguide has minimum transmission losses per unit of length, accompanied by minimum leaks. It is of interest to utilize these properties by providing microwave sources within a standard waveguide.
In order to obtain a high output power from an IMPATT diode it is desirable to use a maximum junction surface area. Thus, the diode impedance is one or two orders of magnitude below the characteristic impedance of the transmission waveguide. Therefore, an impedance transformation ratio is necessary between these two impedances of very different values. In a typical case, it is necessary to pass from an impedance of several hundred ohms to an impedance of a few ohms to obtain correct matching. This problem is exacerbated when it is necessary to combine a plurality of diodes in parallel.
Matching methods used in centimeter frequency bands are available. However, they cannot be extrapolated into millimeter frequency bands due to the mechanical precision required and the difficulties of fitting into a waveguide with a very small cross-section.
T.MISAWA has already proposed in the Journal IEEE Trans. MTT, November 1970 a microwave oscillator construction comprising a single IMPATT diode welded to the largest inner wall of a waveguide. This diode is covered by a disk kept under a mechanical pressure. The radius of this disk is approximately a quarter of the wavelength and the part of the disk in contact with the diodes has a slight conicity.
A different construction was proposed by G. CACHIER et al. (1977 ISSC Conf. Digest pp. 126-127) in which the diode is welded to a metal connecting piece and the opposite face of the said diode receives a gold ball. The assembly constituted by the connecting piece, the diode and the ball is surrounded by an annular torus made from a dielectric material. The free space between this assembly and the annular torus is filled with a dielectric resin. Finally, the upper face of this assembly receives a metal disk deposited in the form of a metal coating.
Neither of these two constructions is suitable for the combining of a plurality of diodes, due to their axial configuration. Moreover, these constructions are not matched to the rectangular characteristic of a waveguide.